We present a new constraint system with equational and membership constraints over infinite trees. It provides for complete and correct satisfiability and entailment tests and is therefore suitable for the use in concurrent constraint programming systems which are based on cyclic data structures.Our set defining devices are greatest fixpoint solutions of iregular systems of equations with a deterministic form of union. As the main technical particularity of the algorithms we present a novel memorization technique. We believe that both satisfiability and entailment tests can be implemented in an efficient and incremental manner.